Muscle & peripheral nerve nontumor

Inflammatory myopathies

Polymyositis



Minor changes: 4 October 2021

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PubMed Search: Muscle [TI] polymyositis[TI] free full text[sb]

Ashley M. Holloman, M.D.
Carrie A. Mohila, M.D., Ph.D.
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Cite this page: Holloman AM, Mohila CA. Polymyositis. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/musclepolymyositis.html. Accessed October 15th, 2021.
Definition / general
Essential features
  • Myopathic features with endomysial inflammatory infiltrate of predominantly CD8+ T cells surrounding and invading nonnecrotic muscle fibers expressing MHC I antigen (J Neuromuscul Dis 2018;5:109)
  • Diagnosis of exclusion made after all other inflammatory disorders have been ruled out, including other idiopathic inflammatory myopathies (dermatomyositis, inclusion body myositis, antisynthetase syndrome and immune mediated necrotizing myopathy), toxin / drug induced myopathies and muscular dystrophies
Terminology
  • Polymyositis, idiopathic inflammatory polymyositis, idiopathic polymyositis, pure polymyositis
ICD coding
  • ICD-10:
    • M33.20 - polymyositis, organ involvement unspecified
    • M33.21 - polymyositis with respiratory involvement
    • M33.22 - polymyositis with myopathy
    • M33.29 - polymyositis with other organ involvement
Epidemiology
Sites
Pathophysiology
Etiology
Clinical features
  • Subacute onset of proximal muscle weakness
  • Extramuscular manifestations include systemic symptoms (e.g. fever, weight loss) and Raynaud phenomenon
  • May be associated with other connective tissue disorders such as rheumatoid arthritis and Sjögren syndrome
  • May also be associated with cardiac and pulmonary dysfunction including arrhythmias and interstitial lung disease, respectively
  • References: Lancet 2003;362:971, Clin Exp Rheumatol 2014;32:188, RMD Open 2017;3:e000507
Diagnosis
  • Diagnosis of exclusion after other inflammatory myopathies have been ruled out
  • Diagnosis of definite polymyositis requires the demonstration of CD8+ lymphocytes surrounding and invading nonnecrotic muscle fibers that express MHC class I antigen in the absence of rimmed vacuoles (Neuromuscul Disord 2015;25:268, Neuromuscul Disord 2004;14:337)
  • In the absence of endomysial inflammation, a diagnosis of probable polymyositis can be made if there is ubiquitous expression of MHC class I antigen, in the absence of rimmed vacuoles and dystrophic changes (Neuromuscul Disord 2015;25:268, Neuromuscul Disord 2004;14:337)
  • The European League Against Rheumatism / American College of Rheumatology (EULAR / ACR) Classification criteria may clinically help distinguish major subgroups of idiopathic inflammatory myopathies and distinguish them from mimicking conditions (Arthritis Rheumatol 2017;69:2271)
    • Score based on age of onset, muscle weakness, skin manifestations, dysphagia or esophageal dysmotility, laboratory measurements (anti-Jo1 autoantibodies; elevated serum creatine kinase [CK], lactate dehydrogenase [LDH], aspartate aminotransferase [AST] or alanine aminotransferase [ALT]), with or without muscle biopsy findings
Laboratory
Radiology description
Prognostic factors
Case reports
Treatment
  • Oral glucocorticosteroids are first line therapy
  • Intravenous (IV) steroids, immunosuppressants, intravenous immunoglobulin (IVIG) and immunomodulators may be considered in cases which do not respond to initial therapy (Arthritis Rheumatol 2018;70:1532)
Gross description
  • Skeletal muscle usually submitted as 1 large piece but may also be submitted in small, multiple pieces
  • Gross findings are nonspecific
Microscopic (histologic) description
  • Myopathic changes including myofiber size variation with small rounded myofibers and increased internalized nuclei
  • Endomysial lymphocytic inflammation surrounding and invading nonnecrotic myofibers
  • Lymphocytic infiltrate may be accompanied by macrophages
  • Degenerating and basophilic regenerating myofibers with large nuclei with prominent nucleoli
  • May see rare necrotic myofibers with phagocytosis
  • Lacks perifascicular atrophy
  • Lacks rimmed vacuoles
  • References: Lancet 2003;362:971, Clin Exp Rheumatol 2014;32:188, RMD Open 2017;3:e000507
Microscopic (histologic) images

Contributed by Carrie A. Mohila, M.D., Ph.D.
Endomysial lymphocytic inflammation Endomysial lymphocytic inflammation

Endomysial lymphocytic inflammation

Endomysial lymphocytic infiltrate

Endomysial lymphocytic infiltrate

CD3+ lymphocytic infiltrate

CD3+ lymphocytic infiltrate

Predominantly CD8+ lymphocytic infiltrate

Predominantly CD8+ lymphocytic infiltrate

MHC1

MHC1

Positive stains
Negative stains
Electron microscopy description
  • Lack of rimmed vacuoles and tubuloreticular inclusions
Sample pathology report
  • Skeletal muscle, left vastus lateralis, biopsy:
    • Inflammatory myopathy compatible with polymyositis (see comment)
    • Comment: This biopsy shows myopathic changes associated with a lymphocytic inflammatory infiltrate of CD8 positive T cells invading nonnecrotic muscle fibers that express MHC class I antigens. There is no evidence of perifascicular atrophy, capillary loss or tubuloreticular inclusions on electron microscopy to suggest dermatomyositis. Rimmed vacuoles are not present. There are no dystrophic features. In the correct clinical setting, this biopsy is compatible with polymyositis. Correlation of biopsy findings with clinical features and myositis specific autoantibodies is required to exclude other idiopathic inflammatory myopathies.
Differential diagnosis
Board review style question #1

Which of the following statements is true regarding the skeletal muscle biopsy image shown above?

  1. CD20+ B cells are the predominant inflammatory component
  2. CD8+ T cells are the predominant inflammatory component
  3. Histologic findings are classic for dermatomyositis
  4. IVIG is the first line therapy
  5. Presence of rimmed vacuoles favors polymyositis
Board review style answer #1
B. CD8+ T cells are the predominant inflammatory component

Comment Here

Reference: Polymyositis
Board review style question #2
Which of the following statements is true regarding the diagnosis of polymyositis?

  1. Histologic findings alone are characteristic and diagnostic of disease
  2. Patients often present with skin rash
  3. Polymyositis is a diagnosis of exclusion
  4. Polymyositis is associated with a specific autoantibody
  5. Sarcolemmal staining for MHC class I antigen is specific for polymyositis
Board review style answer #2
C. Polymyositis is a diagnosis of exclusion

Comment Here

Reference: Polymyositis
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